This invention relates to a method for the manufacture of an interdigital periodic structure device by the technique of molecular-beam epitaxial crystal growth, and more particularly to a method for the manufacture of an interdigital periodic structure device which is usable as a photodetecting device capable of highly efficient photoelectric conversion of beams of even long wavelength or as an FET.
The photodetector for converting radiant energy such as that of solar rays into electric energy possesses a hetero-junction structure of semiconductor materials or a PIN structure incorporating an interposed insulating film. On receiving a photon flux whose energy is greater than the energy gap in the forbidden band of the semiconductor materials, the semiconductor generates electrons in the conduction band and holes in the valence hand, and the electrons or holes are respectively transferred to the opposite electrodes, there to produce a terminal voltage. The photodetector, therefore, produces no terminal voltage when the incident energy is smaller than the energy gap in the forbidden band. Even when the incident energy is greater than the energy gap, the device's efficiency in photoelectric conversion is lowered if the wavelength in the peak in spectrum of the light deviates greatly from the energy gap. The solar battery using p-n junctions of single crystal Si elements manifests its maximum sensitivity when the wavelength of the incident light falls within the range of from 0.7 to 0.8 .mu.m. It fails to operate with its highest efficiency, therefore, because in the spectrum of sunlight, the band of the maximum energy falls in the neighborhood of 0.5 .mu.m. None of the Si photodetectors so far introduced have been able to provide efficient photoelectric conversion of light having wavelengths of 1 .mu.m or more.
The semiconductor photodetector used in the optical communications are made from ternary or quaternary compound semiconductors so as to exhibit the maximum sensitivity to wavelengths of from 1.3 to 1.6 .mu.m, the range in which the light transmittion loss of the optical fibers is minimized. They nevertheless have a disadvantage that they entail lattice defect or dislocation due to the lattice mismatching. From the standpoint of the processing of signals in the optical communications, the optical information transmitted through the optical fibers is required to undergo electrical arithmetic processing and control. Such an electric IC technique has already been established with respect to Si elements. A breakthrough which would enable lights of wavelengths exceeding 1 .mu.m to be effectively detected by use of Si elements would contribute much to improving the compatibility of optical information with conventional electric IC elements.
G. H. Dohler of the Max-Plank Institute in West Germany has published a report purporting that it is theoretically possible to effect variation in the energy gap of the forbidden band by use of a doping superlattice structure. (J. Vac. Sci. Tech., 16 (3), pp. 851, 856 1979). This means that by use of a device having thin films of P-N type semiconductor in a periodic structure, an effective band gap of a smaller width than the band gap proper to the single-junction semiconductor can be materialized and that by adopting this superlattice structure in the photodetector, this photodevice can be made effectively operable with the beam of a greater wavelength than the highest possible wavelength with which the conventional single-junction structure devices have been operable.
Molecular beam epitaxy (MBE) is the optimum method available for producing such a superlattice structure. It is capable of forming thin crystal films free of crystal defects. By this method, it becomes feasible to obtain heterojunctions having a satisfactory lattice matching property. Particularly noteworthy is the fact that in this method the thickness of epitaxial layers and the distribution of impurities concentration can be controlled to an accuracy of some tens of Angstroms and the film thickness control can be effected on the order of 5 .ANG.. ("Growth of Periodic Structures by the Molecular-Beam Method" by A. Y. Cho, Applied Physics Letters, Vol. 19, No. 11, 1st Dec. 1971, pp. 467-468)
A technique for producing this superlattice structure by use of the MBE has already been established. In spite of this established technique, no method has so far been developed which permits actual manufacture of photodetectors such as solar batteries. One seemingly feasible method would comprise forming a superlattice of a PN periodic structure, then causing diffusion or ion implantation of N.sup.+ impurities in one of the opposite end faces perpendicular to the P-N junction interface and of P.sup.+ impurities in the other end face, and finishing the entire lattice in the interdigital shape. However, since the finishing molding is performed at elevated temperatures, the impurities doped into the layered films are diffused so much as to make production of the desired structure infeasible.